DESCRIPTION

The function new_fieldtype creates a new field type usable for data
validation. You supply it with field_check, a predicate to check the
validity of an entered data string whenever the user attempts to leave a field.
The (FIELD *) argument is passed in so the validation predicate can see the
field's buffer, sizes and other attributes; the second argument is an
argument-block structure, about which more below.

You also supply new_fieldtype with char_check,
a function to validate input characters as they are entered; it will be passed
the character to be checked and a pointer to an argument-block structure.

The function free_fieldtype frees the space allocated for a given
validation type.

The function set_fieldtype_arg associates three storage-management functions
with a field type.
The make_arg function is automatically applied to the
list of arguments you give set_field_type when attaching validation
to a field; its job is to bundle these into an allocated argument-block
object which can later be passed to validation predicated.
The other two hook arguments should copy and free argument-block structures.
They will be used by the forms-driver code.
You must supply the make_arg function,
the other two are optional, you may supply NULL for them.
In this case it is assumed
that make_arg does not allocate memory but simply loads the
argument into a single scalar value.

The function link_fieldtype creates
a new field type from the two given types.
They are connected by an logical 'OR'.

The form driver requests REQ_NEXT_CHOICE and REQ_PREV_CHOICE assume
that the possible values of a field form an ordered set, and provide the forms
user with a way to move through the set.
The set_fieldtype_choice
function allows forms programmers to define successor and predecessor functions
for the field type.
These functions take the field pointer and an
argument-block structure as arguments.

RETURN VALUE

The pointer-valued routines return NULL on error.
They set errno according to their success:

E_OK

The routine succeeded.

E_BAD_ARGUMENT

Routine detected an incorrect or out-of-range argument.

E_SYSTEM_ERROR

System error occurred, e.g., malloc failure.

The integer-valued routines return one of the following codes on
error: